1. Field of the Invention
The present invention relates to a light-emitting element having an anode, a cathode, and a layer containing an organic compound in which luminescence can be obtained by applying an electric field (hereinafter, referred to as a “layer containing a light-emitting substance”), and specifically to a light-emitting element which can achieve long term stabilization.
2. Description of the Related Art
A light-emitting element (also referred to as an organic or organic-inorganic hybrid electroluminescent element) and a solar battery are given as an example of a photoelectronic device using an organic semiconductor material as a functional organic material. These devices utilize electrical properties (carrier transportability) and optical properties (light absorption or light emission) of organic semiconductor materials, and specifically, the light-emitting element shows remarkable development.
A light-emitting element is formed by sandwiching a layer containing a light-emitting substance between a pair of electrodes (an anode and a cathode). As the light-emitting mechanism, it is said that a hole injected from an anode and an electron injected from a cathode are recombined at the luminescence center in the layer containing a light-emitting substance to form a molecular exciton when voltage is applied between the both electrodes and energy is discharged to emit luminescence when the molecular exciton returns to a ground state. A singlet excitation state and a triplet excitation state are known as an excited state, and it is considered that luminescence can be obtained by undergoing either excited state.
Conventionally, it has been attempted to increase luminous efficiency of a light-emitting element or to extend the life thereof. Although luminescence having high luminance can be obtained, there is a defect that an emission lifetime is short. As these factors, it can be considered that bondability between a metal electrode and a film during lamination, for example, an organic compound layer is not good due to electric conduction for a long time. Therefore, various considerations are made to increase luminous efficiency of a light-emitting element or to extend the life thereof.
Incidentally, a light-emitting element using an alkali metal having a low work function is know as a light-emitting element with good luminous efficiency so far (for example, Reference 1: Japanese Patent Laid-Open No. Hei 5-121172). Although the light-emitting element using an alkali metal has favorable efficiency, the alkali metal needs to be treated with religious care since it is easily oxidized; therefore, there is a problem that it is difficult to use the alkali metal for manufacturing a light-emitting element.
In addition, a light-emitting element which interpose a thin film insulating layer using metal fluoride or the like easy to be dealt with in an interface between an organic and a cathode is known, too (for example, see Reference 2: L. S. Hung, two others, Applied Physics Letters, vol. 70, pp. 152-154 (1997)). However, although luminous efficiency can be improved compared with a light-emitting element using an MgAg alloy known as a cathode with a low work function, there is a problem that the adhesiveness with an organic thin film is poor and stable luminescence cannot be easily obtained.